In some Three-Dimensional Integrated Circuits (3DIC), device dies are first bonded to an interposer, which is further bonded to a package substrate to form a package. The heat generated in the device dies during their operation needs to be dissipated. In the conventional structures, to dissipate the heat, the substrates of the device dies are attached to a metal lid, which helps dissipate heat, and also acts as a stiffener. Accordingly, the heat generated in the device dies is spread to the metal lid. A heat sink may be attached to the metal lid to further dissipate the heat conducted to the metal lid.
The attachment of the device dies to the metal lid is through a Thermal Interface Material (TIM), which may include an epoxy-based material. Since the TIM has relatively low thermal conductivity, it is preferred that the TIM is thin so that the TIM does not introduce too much thermal resistance between the device dies and the metal lid.